Lubrication



United States Patent 3,255,621 LUBRICATION Ernest 0. Ohsol, Wilmington,Del., assignor to Haveg Industries, Inc., a wholly-owned subsidiary ofHercules Powder Company, New Castle, Del., a corporation of DelawareFiled Aug. 16, 1963, Ser. No. 302,674 9 Claims. (Cl. 72-42) Thisinvention relates to fluorocarbon polymers.

In the forming of refractory metals by extrusion through dies at hightemperatures lubrication of the die is a very critical problem. Thepressures and temperatures encountered, e.g. a temperature of 3500 F.(1927 C.), are extremely high and conventional die lubricants such asmolten glass have insuflicient film strength and too low a viscosity tobe satisfactory lubricants for formingrefractory metals such astungsten, columbium, titanium and molybdenum and the like.

Previously employed solid lubricants for refractory materials provideinsufficient lubrication at low temperatures, leave abrasive residuesafter high temperature exposure an-d are physically removed before thelubrication function is complete.

Graphite powder and molybdenum sulfide have been tried in paste form,but are readily displaced. Furthermore, at very high temperatures carbonand graphite lose their adsorbed film and become non-lubricating.

Teflon inserts by themselves are unsatisfactory because they have poorresistance to mechanical displacement. Additionally the Teflon tends tobe volatilized away too soon.

Accordingly it is an object of the present invention to prepare asuperior type of ablative lubricating die insert.

A further object .is to prepare novel lubricants.

Yet another object is to prepare improved rocket noz zles.

Still further objects and the entire scope and applicability of thepresent invention will become apparent from the detailed descriptiongiven hereinafter; it should be understood, however, that the detaileddescription and specific examples, while indicating preferredembodiments of the invention, are given by way of illustration only,since various changes and modifications within the spirit and scope ofthe invention will become apparent to those skilled in the art from thisdetailed description.

It has now been found that these objects can be attained by preparingperfluorocarbon polymer impregnated silica or glass cloth. Theimpregnated cloth is then molded into an ablative lubricating die insertor a rocket nozzle.

The invention will be best understood in connection with the drawingswherein:

FIGURE 1 is a view of an apparatus for'extrusion of a refractory metal;and

FIGURE 2 is a sectional view of a polytetrafluoroethylene impregnatedglass cloth.

The polytetrafluoroethylene impregnated glass or silica cloth is old perse and can be prepared by any convenient prior art process such as thatshown in Sanders Patent 2,539,329 or Fay Patent 2,843,502.

T 0 give improved results it is frequently desirable to incorporate 4 to8% of molybdenum disulfide based on the total weight of the Teflon,cloth and molybdenum disulfide. The molybdenum disulfide is readilyincorporated by including it in powder form in the Teflon dispersionapplied to the cloth, e.g. there can be employed a dispersion of 40parts Teflon (polytetrafluoroethylene) and parts of molybdenum disulfidein 55 parts of water containing 3.6 parts of Triton X-100 (octyl phenylpolyglycol ether) to impregnate glass cloth to obtain a productcontaining 40% Teflon, 5% molybdenum disulfide and 55% glass cloth byweight.

ice

preferably dried at relatively high temperatures, e.g.. 300 C., whichare within 50 C. of the fusion temperature of Teflon. Drying can even bedone at the sintering temperature or above, eLg. 330 C.

T 0 form the die insert a well bonded dense laminate of the Teflonimpregnated glass cloth (or silica cloth) with or without the molybdenumdisulfide is prepared. This can be accomplished by stacking or wrappingthe Teflon impregnated cloth around a mandrel and then sintering theproduct at a high pressure, e.g. 300 to 2,000 psi. at a temperatureabove the sintering temperature (327 C.) for Teflon. Thus the Teflon-60%carbon cloth wrapped around a mandrel to form several thicknesses ofcloth can be heated to 350 C. at a pressure of 1000 psi. and then cooledto room temperature under pressure.

Instead of wrapping the Teflon impregnated cloth around the mandrel aplurality of layers can be laminated together to form the die insert bycompressing them lightly, e.g., 100 psi. then sintering the product,e.g., at

340 C. with no applied pressure and then finally pressing at :highpressure, e.g., 500 psi. and then machined to dimensions.

The products of the present invention are characterized by absence ofvoid spaces, high strength and dimensional stability.

The glass or silica cloth inserts of the present invention can contain25 to 75% Teflon (preferaby 30 to 65%), and 75 to 25% glass or silicacloth (preferably '70 to 35%). If molybdenum disulfide is employed it ispresent in minor amount, e.g. 4 to 8%. p

The glass or silica phase provides the high temperature lubrication, egat temperatures of 900 to 2000 C. to

' an excellent degree, having previously been kept cool by thevaporizing polytetrafluoroethylene. Essentially a solid-liquidlubrication system is employed.

Typical examples of refractory metals which can be formed with the aidof the ablative lubricating die insert or plug of the present inventionare tungsten, columbium, tantalum, titanium, molybdenum, uranium,thorium and zirconium and alloys such as certain stainless steels.

In place of employing the preferred material, polytetrafluoroethylene,other fluorocarbon polymers can be used such as polyhexafluoropropyleneand copolymers of tetrafluoroethylene with hexafluoropropylene havingweight ratios of :25 or 30:70 or 34:66 or 19:90 or 1:2 or 4:3 or 23.8:30or :15.

Referring to FIGURE 1 of the drawing there is provided a frusto-conicalablative lubrication die insert 2 -made of 40% Teflon-60% glass cloth.The cloth had a thread count per inch of 60 x 47, yarn size 900 /2,ounces per square yard 1.43. The insert had the dimensions shown inFIGURE 1. Thus at its front end or inlet 4 it had a diameter of 3.05inches and tapered to its rear surface or outlet 6 where it had adiameter of 1.00 inch. The clearance at the inlet was 2.414 inches. Theside 8 of the trusto cone made a 30 angle with the vertical axis and theside 10 made a 45 angle with the horizontal axis. Thus the inner andouter sides of the frusto cone embraced an angle of 15. The Teflon-glasscloth composite was preferentially made by arranging the laminates ofthe glass cloth in acircumferential pattern coaxial with the center lineof the insert. Successive layers of the lamination had a slight angle tothe inside surface, e.g. 10. It is also possible as previously indicatedto make a block of substantially flat laminations of the Teflon andglass cloth and to machine the die insert from such a thick block.

The die insert is fitted into a larger hardened steeldie 12. The die 12similarly has a conical entrance section tapering from 3.05 inches atthe inlet to 1 inch at the outlet. The throat 14 of the die is only afraction of an inch long and is immediately expanded to an outlet 16diameter of 1.5 inch over a distance of 1 inch. There is also provided abillet 18 of the refractory metal, e.g. tungsten. The billet has anoutside diameter close to the 3 inch entrance diameter of the die andissues with a diameter of about 1 inch. The entering portion 20 of thebillet is chamfered, e.g. with a 35 chamfer, to facilitate entrance intothe die. There is also provided a plunger 22 to force the billet 18 intothe die insert 2 and die 12.

In operation the hot billet, cg. tungsten at 1800 C. is rammed throughthe die at a pressure of 100,000 p.s.i. In an extrusion of a 10 or 20foot metal rod by this means, an operation which lasts only a fewseconds, the Teflon in the die insert will first act as a lubricant, butwill be rapidly vaporized upon contact with the hot metal surface. Thefilm of vaporizing Teflon helps to provide lubrication and keep thesurface of the metal clean. It also helps to keep the outer portions ofthe die insert and the metal die itself cool by virtue of its heat ofablation. At the same time, the layers of glass cloth remaining providea solid-liquid type of lubrication.

FIGURE 2 shows a laminate of layers of glass cloth 24 impregnated andbonded with Teflon 26 to form a die insert. As indicated, silica cloth,e.g. 9099% silica, can be used in place of glass cloth.

I claim:

1. In a method of extruding a refractory metal workpiece at hightemperature from an extrusion chamber having an opening through whichthe workpiece is extruded the improvement comprising providing anablative lubricating die insert in said die, said insert comprising acloth of the group consisting of glass cloth and silica clothimpregnated with a perfluorocarbon polymer, heating the workpiece andintroducing it into the extrusion chamber, extruding the workpiecethrough said die insert at a temperature and pressure suificient tovapor- 4 ize said perfiuorocarbon polymer to provide lubrication and toexpose and melt said cloth to provide solid-liquid lubrication of theworkpiece.

2. A process according to claim 1 wherein the extrusion is carried outat a temperature of at least 900 C.

3. A process according to claim 2 wherein a minor amount of molybdenumdisulfide is incorporated in said polymer.

4. A process according to claim 1 wherein the polymer is selected fromthe group consisting of polytetrafluoroethylene, polyhexafluoropropyleneand copolymers of tetrafluoroethylene and hexafiuoropropylene and theextrusion temperature is at least 900 C.

5. A process according to claim 1 wherein the polymer.

is polytetrafluoroethylene.

6. A process according to claim 5 wherein the extrusion temperature isat least 900 C.

7. A process according to claim 6 wherein there is incorporated with thepolytetrafluoroethylene a minor amount of molybdenum disulfide.

8. A process according to claim 1 wherein the polymer ispolytetrafluoroethylene, the cloth is glass cloth and the extrusiontemperature is at least 900 C.

9. A process according to claim 1 wherein the polymer ispolytetrafluoroethylene, the cloth is silica cloth and the extrusiontemperature is at least 900 C.

References Cited by the Examiner UNITED STATES PATENTS CHARLES W.LANHAM, Primary Examiner. MICHAEL V. BRINDISI, Examiner.

E. D. OCONNOR, H. D. HOINKES,

Assistant Examiners.

1. IN A METHOD OF EXTRUDING A REFACTORY METAL WORKPIECE AT HIGHTEMPERATURE FROM AN EXTRUSION CHAMBER HAVING AN OPENING THROUGH WHICHTHE WORKPIECE IS EXTRUDED THE IMPROVEMENT COMPRISING PROVIDING ANABLATIVE LUBRICATING DIE INSERT IN SAID DIE, SAID INSERT COMPRISING ACLOTH OF THE GROUP CONSISTING OF GLASS CLOTH AND SILICA CLOTHIMPREGNATED WITH PERFLUOROCARBON POLYMER, HEATING THE WORKPIECE ANDINTRODUCING IT INTO THE EXTRUSION CHAMBER, EXTRUDING THE WORKPIECETHROUGH SAID DIE INSERT AT A TEMPERATURE AND PRESSURE SUFFICIENT TOVAPORIZE SAID PERFLUROROCARBON POLYMER TO PROVIDE LUBRICATION AND TOEXPOSE AND MELT SAID CLOTH TO PROVIDE SOLID-LIQUID LUBRICATION OF THEWORKPIECE.